X-ray imaging is a valuable tool for research, industrial applications, and medical applications. By irradiating a target object with x-ray radiation, and by detecting the x-rays transmitted through the object, an image of the internal structure of the object can be obtained.
Such an image allows the identification of those portions of the object which relatively more attenuate the passage of x-rays, and those which relatively less attenuate the passage the x-rays. In general, denser materials, and those comprising a high proportion of high-atomic-number atoms or ions, will tend to impede the passage of x-rays to a relatively greater degree. Furthermore, the longer the total path length travelled by the x-rays in the target object, the greater the degree of attenuation. Therefore, in addition to providing structural information, x-ray imaging can provide information about the composition of the object.
Additionally, by rotating the target object relative to the source-detector system, or vice-versa, acquiring a sequence of x-ray images at different angles about the object, and applying computer reconstruction techniques, a 3D volume map of the object can be determined. Such a map allows reconstruction of those volumetric portions of the object which attenuate x-rays to a greater or lesser extent, and thus allows information about the internal structure and composition of an object to be determined in 3D. Such 3D reconstruction is termed computerised tomography, or CT imaging.
Such x-ray imaging techniques are of particular importance in non-destructive testing of industrial products and research specimens. For example, the imaging of turbine blades allows casting defects to be determined, while the imaging of archaeological artefacts allows the structure and composition of an artefact to be determined, even when the object is corroded or encased in sedimentary deposits. For example, such techniques have been invaluable in the determination of the internal structure of the ancient Corinthian analogue computer known as the Antikythera mechanism, even through extensive mineral deposits.
However, the property that enables x-rays to be of advantage in analysing the internal structure of an object, namely their partial attenuation by dense matter, also presents a technical limitation as to their utility. More particularly, if the object is dimensionally large or contains a large amount of radiodense or radiopaque material, being material which presents a relatively high attenuation per unit path length of x-ray radiation, the x-ray beams having passed through the object may be attenuated to such a degree that the contrast or signal-to-noise in the recorded image is poor, and consequently the internal structure or composition cannot be reliably determined.
Where attenuation is only moderate, increasing the total x-ray flux through the object can yield improvements in signal-to-noise and contrast at the detector. However, where the object is so large or so radiodense that a high proportion of the x-rays incident on the object are unable to fully transit the object, but are rather absorbed within the object, a different solution is needed.
The distance an x-ray photon will characteristically penetrate before absorption, the “penetration depth” increases with x-ray photon energy. Therefore, the generation of x-ray sources of high x-ray photon energy, especially to 300 keV or greater, enables the useful x-ray imaging of larger and denser objects. However, commercially practical x-ray sources of suitably high energy have not been produced.
Accordingly, there is a need in the art for an x-ray source which can operate at energies of up to 500 keV, and beyond, and which is suitable for use in commercial and research x-ray applications such as computerised tomography (CT).